linux-kernel - Re: [RFC] x86: xsave/xrstor support, ucontext

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Message-ID: <18485.13215.945.950687@harpo.it.uu.se>
Date:	Thu, 22 May 2008 10:49:34 +0200
From:	Mikael Pettersson <mikpe@...uu.se>
To:	Roland McGrath <roland@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mikael Pettersson <mikpe@...uu.se>,
	Andi Kleen <andi@...stfloor.org>, mingo@...e.hu,
	tglx@...utronix.de, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, drepper@...hat.com,
	Hongjiu.lu@...el.com, linux-kernel@...r.kernel.org,
	arjan@...ux.intel.com, rmk+lkml@....linux.org.uk, dan@...ian.org,
	asit.k.mallick@...el.com
Subject: Re: [RFC] x86: xsave/xrstor support, ucontext_t extensions

Roland McGrath writes:
 > > The biggest potential problem with this that I see is that relying on 
 > > CPUID can mess with certain virtualization solutions.  Another option to 
 > > accomplish the same thing would be to have a system call (preferrably a 
 > > prctl, since it is at least in theory personality-dependent) to query 
 > > what information is included in the fpstate data - since it will always 
 > > be the same for any particular kernel.
 > 
 > It's easy and cheap to add an indicator to the vDSO for this.

For our user-space virtualisation code a plain system call,
like prctl(), would be strongly preferable. Right now I
can't say whether a vDSO solution would work at all for us,
but a system call solution would be no problem at all.

/Mikael
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